Exclusive hard diffraction at HERA DVCS and Vector

Exclusive (hard) diffraction at HERA (DVCS and Vector Mesons) P. Marage Université Libre de Bruxelles 1

The HERA harvest initial state : real and virtual photons final state VM : g, r, f, J/y, y(2 s), U final state baryonic syst. : p, p. diss. MV 0 – 10 Ge. V Q 2 0 – 80 Ge. V 2 W ≈ √(Q 2/x) 30 – 300 Ge. V |t| = |pt, miss|2 0 – 30 Ge. V 2 photoproduction & DIS small (< 0. 5 – 3 Ge. V 2) & large |t| (2 < |t| < 10 -30 Ge. V 2) > 30 H 1 + ZEUS published exp. papers most recent results (2008 -2009) - DVCS ZEUS (LPS) + H 1 - r photoprod. H 1 - r and f electroprod. H 1 - U photoprod. ZEUS - real g at large |t| H 1 P. Marage DESY-08 -132 + H 1 -prelim-09 -014 H 1 -prelim-09 -016 DESY-09 -093 (to appear soon) DESY-09 -036 DESY-08 -077 Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 2

Context QCD frameworks collinear factorisation (GPD) proton rest frame factorisation (dipole approach) scale - long. amplitudes - transv. amplitudes Content DVCS ds / d. Q 2, d. W, dt (small t) VM helicity amplitudes elast. / proton diss. ; Regge factorisation ds / d. W, dt (large t); helicity amplitudes P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 3

Event selection and backgrounds Photoprod. (Q 2 ~ 0) / electroprod. (Q 2 > 1 Ge. V 2) : absence / presence of electron cand. VM selection : 2 tracks + inv. mass selection elastic events : LPS (ZEUS) or no tag in forward detectors (H 1, Zeus) proton dissociation : tag events, |t| <3 Ge. V 2 |t| < 0. 5 Ge. V 2 backgrounds: elastic <-> p. diss. events other VM’s (r’) continuum r U P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 4

DVCS P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 5

Q 2 dependence LPS GPD and dipole models GPD take into account skewing but kin. skewing is not sufficient P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 6

Re / Im (BCA) interference with Bethe-Heitler → access to Re contributions Beam charge asymmetry (e+ vs. e-) in agreement with dispersion relation analysis W, t dependences; effective Regge trajectory : see below P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 7

2 Q P. Marage dependences (VM) Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 8

r and f (el. and p. diss. ) Excellent ZEUS-H 1 agreement on total cross sections (also for f with ZEUS p. diss. bg. subtraction as for r) P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 9

r and f – polarised cross sections NB fixed W, varying x P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 10

(approximate) universality Cross sections scaled by quark charge content Very strong Q 2 dependence approximate universality as a function of Q 2+M 2 P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 11

W dependences P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 12

Photoproduction : mass dependence soft stot r, w, f hard J/Y U (skewing, Re GPD) P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 13

DVCS and electroprod. : Q 2 dependence DVCS J/y r see below – effective Regge trajectory P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 14

t dependences P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 15

t dependences elast. DVCS (LPS) p. diss. r f DVCS (p. diss. ) P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 16

b slopes elastic p. dissoc. optical model - b decrease with dipole size (Q 2, mq) - universal scale dependence for m 2 > 5 Ge. V 2 m 2 = (Q 2 + M 2)/4 for VM m 2 = Q 2 for DVCS - b(r, f at large Q 2) -> b (J/y) -> small room for VM form factor b. VM P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 17

bel – bpd and Regge factorisation Vertex (“Regge”) factorisation Ø Ø bel. – bp. diss. ~ cte (? ) ~ 5. 5 for r, f = 4 for DVCS (m 2 = Q 2) = 3. 5 for J/y -> factorisation for m 2 > 5 Ge. V 2 ? P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 18

Effective Regge trajectory P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 19

Effective trajectory ü W dependence s~Wd ü d = 4 (a(t) – 1) a(t) = a(0) + a’ t shrinkage of diffractive peak W – t correlation t dependence as a function of W P. Marage W dependence as a function of t Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 20

Shrinkage Xx Put rho photoprod. r electroprod. P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 21

a’ slope hadron-hadron inter. : a‘ = 0. 25 Ge. V-2 H 1 DVCS : a‘ = 0. 03 ± 0. 09 ± 0. 11 Ge. V-2 NB J/y 2 < |t| < 30 Ge. V 2 photoprod. , a‘ = - 0. 02 ± 0. 01 Ge. V-2 (H 1 + ZEUS) P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 22

a. P(0) m 2 = (Q 2 + M 2)/4 for VM m 2 = Q 2 for DVCS hardening with Q 2 and mass (dipole size) light VM : getting similar to J/y for m 2 larger than about 5 Ge. V 2 P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 23

Remark on scales P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 24

Helicity amplitudes P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 25

spin density matrix elements 3 angles describe VM production and decay → 15 spin density matrix elements bilinear combinations of helicity amplitudes Tlr, lg (NPE is assumed) SCHC T 00 T 11 single flip T 01 T 10 double flip T-1 1 P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 26

spin density matrix elements (Q 2) (r) 5 SCHC elements compared to GPD calculations Other elements (dashed lines) compatible with 0 or small) except ~ T 01 T 00* several models (GPD, unintegrated kt, dipole + saturation) can reproduce general features of SCHC amplitudes + hierarchy but not details P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 27

R(Q 2) ; Q 2 / M 2 p. QCD : formally R ~ Q 2 / M 2 scaling plot (Q 2 / M 2 V) - scaling OK - lower than 1 - damping at large Q 2 P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 28

R(W) No W dependence of s. L / s. T P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 29

R (t) ; b. L - b. T Q 2 > 5 Ge. V 2 H 1 Q 2 > 5 Ge. V 2 : b. L - b. T <0 1. 5 s (stat. + syst. ) also t dependence of T 11 / T 00 – see below indication that transverse size of dipoles from transverse photons is larger than from longitudinal photons P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 30

R (m) (r) Decrease of R(m) is qualitatively consistent with formal p. QCD calculations R ~ Q 2 / M 2 if M is diquark / dipion mass rather than resonance mass Support to models (MRT) with small relevance of the VM WF P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 31

amplitudes ratios (t, Q 2) (r) Global fit of 15 SDME -> 4 amplitude ratios (supposed to be purely imaginary) Q 2 < 5 Q 2 > 5 (Ivanov-Kirschner) ü Q 2 dependences (higher twist) of all amplitude ratios ü t dependences of helicity flip amplitudes T 01/T 00 NB double flip amplitude -> gluon polarisation Ø t dependence of T 11 / T 00 - cf. R(t) : different slopes for L and T – here 3 s effect P. Marage T 10/T 00 Exclusive hard diffraction at HERA (DVCS and VM) T-11/T 00 EDS’ 09 – CERN 29/06 -03/07/2009 32

SCHC amplitudes phases (Q 2) (r, f) phase between T 00 and T 11 ≠ 0 phase difference decreases with Q 2 NB phase difference related to different values of r = Re / Im, which are related through dispersion relations to the W evolutions -> indication of different W evolution of transverse and longitudinal ampltudes ? P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 33

Large |t| P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 34

t dependences Power like t dependences for real g, r, J/Y Quote measured powers for photon, rho, jpsi J/Y BFKL running as excluded DGLAP OK (t < MY 2) see also helicity below P. Marage Exclusive hard diffraction at HERA (DVCS and VM) r r BFKL favoured EDS’ 09 – CERN 29/06 -03/07/2009 35

W dependences rise of s with W described by BFKL, not by DGLAP at large |t| J/Y BFKL pred. P. Marage Exclusive hard diffraction at HERA (DVCS and VM) g EDS’ 09 – CERN 29/06 -03/07/2009 36

SDME (r) “naïve” p. QCD predicts large helicity flip, with long. r dominating at large |t| (spin flip ~ t) But SCHC T → T dominates + double flip T → T Reason : chiral odd contribution in g (due to constituent quark mass) → no orbital momentum needed for YT → SCHC BFKL model describes data, except for sign of cf. also t and W dependences P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 37

SDME (J/y) SDME compatible with SCHC, as expected from models and compact J/y WF P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 38

Summary and conclusions P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 39

summary Measurements of kinematic and angular distributions of real g, r and f, J/y, in photo- and electroprod. , small and large |t| Ø DVCS Q 2, W, t dependences; beam charge asymmetry -> Re/Im amplitudes Ø VM at small |t| Q 2, W, t dependences of DVCS, r and f, J/y hard features for Q 2 + M 2 ³ 10 -20 Ge. V 2 helicity amplitudes SCHC + helicity violations Q 2 and t dependences understood in QCD indication of b. L – b. T ¹ 0 cos(d) ¹ 1 effective Regge trajectory apom(0) increases with scale. a‘ < 0. 25 Ge. V-2 GPD and dipole models describe main features, but differences in details Ø VM at large |t| W, t dependences; helicity amplitudes BFKL describes main features, but differences in details P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 40

conclusions Very rich and varied landscape, explored by ZEUS and H 1, complementing lower energy experiments A semi-quantitative understanding is achieved in a QCD framework, in the two complementary approaches of GPD’s and dipole models and, at large |t|, in the BFKL framework Many thanks to the organisers, and to all those to whom I borrowed data – plots – ideas P. Marage Exclusive hard diffraction at HERA (DVCS and VM) EDS’ 09 – CERN 29/06 -03/07/2009 41